Vacuum tube type power amplifiers are used in RF systems and require that both the plate current and grid current be monitored during tune-up sequences and dynamic operation. This information is required by regulatory agencies in both the United States and Canada and imposed upon manufacturers to ensure that power input restrictions outlined by the licensing bodies are adhered to by the operators of the equipment. It is also necessary to have provisions on equipment to monitor the power supply plate voltage. This gives the inspection and licensing bodies some means by which they can calculate the input power by multiplying the input plate current by the input plate voltage to arrive at the input power.
For vacuum tube type RF power amplifiers, the tuning process can be long and tedious. Specifically referring to manually tuned RF power amplifiers, the amplifier is driven by an RF exciter, and both plate current and grid current are simultaneously monitored by two separate meters. As the drive power from the exciter is applied to the amplifier, the plate and the grid currents will rise. The plate current will be required to indicate circuit resonance by displaying a "dipped" plate current value. This value of plate current is unique to the type of power tube in use in the amplifier. At the same time, while power is externally applied to the amplifier, grid current will begin to flow and this will be monitored by a second meter. For maximum power output from the amplifier, there will be a point where a value of plate and grid currents represent the maximum power output in watts, optimum linearity and lowest distortion. These values must be known intimately by the operator in order to avoid overloading or underloading the amplifier when implementing the tune-up procedure.
In order to accomplish this in the past, each of the plate current and grid current have been monitored and the plate and grid currents are known which represent a tuned amplifier. The operator adjusts the load control on the amplifier, which varies both the plate and grid current, and retunes until such time the proper values are achieved. Often this requires adjusting the exciter, as a particular pair of values is tied to the exciting signal.
In addition to the regulatory requirements, it is also desirable to properly tune the amplifier, as operating outside the tuned range shortens the life of the vacuum tube and can lead to distortion which can cause interference in adjacent channels. Therefore, it is highly desirable to be tuned to the proper currents for both the life the amplifier and for the quality of the signal.